Throughout this application, various patents and published patent applications are referred to by an identifying citation. The disclosures of the patents and published patent applications referenced in this application are hereby incorporated by reference into the present disclosure to more fully describe the state of the art to which this invention pertains.
Some products, such as security markings and test strips for the analysis of fluid samples, would benefit from colorless or nearly colorless layers that have strong absorption in the infrared region so that the products can be read by an infrared scanner or camera but do not provide enough of a visible image to be detected by humans or a visible scanner or camera. For example, U.S. Pat. No. 6,316,264 to Corey et al. describes a test strip for determining the presence or concentration of an unknown or a constituent in a liquid test sample, where the test strip comprises an infrared layer that has a detectable response in the infrared region but does not interfere with the response of the test strip in the visible region. Also, for example, U.S. Pat. Nos. 6,381,059; 6,589,451; and 7,151,626, all to Carlson, describe infrared layers for security markings that comprise an aminium radical cation compound and are capable of being detected in the infrared region while being invisible or nearly invisible to the human eye or to detection by a visible scanner.
It would be advantageous if such infrared layers were highly stable in the intensity level of their infrared absorption and in their level of visible coloration to light, heat, and upon extended periods of storage under ambient conditions and, for certain applications that involve direct contact with aqueous fluids such as test strips for fluid samples, were water repellent so the fluids do not cover or degrade the infrared layer.